1. Field of the Invention
The present invention relates to an image forming apparatus for and an image forming method of forming a toner image on a recording medium such as a copy paper, a transfer paper, other general paper, etc.
2. Description of the Related Art
A conventional image forming apparatus such as a printer, a copier machine and a facsimile machine has a structure that a toner image formed on an image carrier such as a photosensitive member and an intermediate transfer medium is transferred onto a recording medium and the toner image is then fixed on the recording medium. To be more specific, for the purpose of executing this transfer process, transfer means such as a conductive and elastic transfer roller and a transfer belt is disposed facing the image carrier. The recording medium is made to move passed this opposed position, namely, a transfer position in synchronization to movement of the image carrier, and a transfer bias is applied upon the transfer means in accordance with the traveling of the recording medium. The toner image carried by the image carrier is consequently transferred on the recording medium. The recording medium to which the toner image has thus been transferred is transported to fixing means and subjected to pressure and heat, thereby fixing the toner image on the recording medium.
In an image forming apparatus which performs double-sided printing, after a toner image is transferred and fixed on one surface of a recording medium in the manner described above, the other surface of the recording medium is subjected to similar transfer/fixing. However, when the both surfaces are subjected to a transfer process and a fixing process under the same conditions, even though printing on one surface (first stage printing) is excellent, printing on the other surface (second stage printing) may not be favorable in some cases. To solve this problem, Japanese Patent Application Laid-Open Gazette No. H2-273771 for example describes to perform double-sided printing while ensuring that a transfer condition for first stage printing is different from that for second stage printing.
The conventional apparatus described above however requires to change a transfer condition between first stage printing and second stage printing, and demands complex transfer control. In the case of an image forming apparatus which forms a color image in particular, toner images in four colors are superposed one atop the other on an image carrier and a resulting color toner image is transferred onto a recording medium. And therefore, it is necessary to strictly control a transfer condition. Hence, in the conventional apparatus described above, it is needed to separately control transfer conditions between first stage printing and second stage printing and strictly control a transfer condition during each printing, which adds to the complexity of the transfer control.
Further, in this type of image forming apparatus, a toner image, which has been formed on an image carrier such as a photosensitive member, is put temporarily on an intermediate transfer medium such as an intermediate transfer belt and an intermediate transfer drum in some cases. While such image forming apparatuses require to secondarily transfer a primary transfer toner image carried by an intermediate transfer medium onto a recording medium, in some of these image forming apparatuses, the toner image is fixed on the recording medium concurrently with the secondary transfer process.
In an apparatus which transfers and fixes at the same time as mentioned above, it is necessary to heat up an intermediate transfer medium, melt toner particles which form a primarily transferred toner image and accordingly fix on a recording medium. Hence, the temperature of thus heated intermediate transfer medium inevitably becomes high. This gives rises to a problem that when the heat gets transmitted to a latent image carrier such as a photosensitive drum and a photosensitive belt, the latent image carrier is adversely affected. To solve the problem, the intermediate transfer medium is heated on the upstream side to a secondary transfer position (transfer/fixing position) (as described in Japanese Patent Application Laid-Open Gazette No. H11-167295 for instance; Patent Document 1). An alternative approach is to dispose a cooling apparatus on the downstream side to the secondary transfer position (Japanese Patent Application Laid-Open Gazette No. 2002-123114; Patent Document 2).
However, the image forming apparatus described in the Patent Document 1 requires merely to dispose heating means on the upstream side to the secondary transfer position and heat up the intermediate transfer medium prior to secondary transfer, but does not provide any particular consideration upon the temperature of the intermediate transfer medium at the secondary transfer position. This may lead to a situation that the temperature of the intermediate transfer medium fails to decrease sufficiently and adversely affects the latent image carrier.
On the contrary, in the image forming apparatus described in the Patent Document 2, since the cooling apparatus is disposed on the downstream side to the secondary transfer position, it is possible to lower the temperature of the intermediate transfer medium and accordingly prevent an adverse influence upon the latent image carrier. However, to dispose such a cooling apparatus gives rise to a problem that a cost of the apparatus and the size of the apparatus increase.
There further is the following problem which is common to the conventional apparatuses described above. That is, in the conventional apparatuses described above, the recording medium is brought into pressure contact with the intermediate transfer medium which has been heated the upstream side to the secondary transfer position without considering the temperature of the intermediate transfer medium at the secondary transfer position at all. Because of this, the pressure contact may drastically change the water content of the recording medium, which may result in inconveniences such as wrinkles and curls.